1. Field of the Invention
The present invention relates to particles and a contrast agent including the particles for optical imaging.
2. Description of the Related Art
A photoacoustic tomography (hereinafter, also referred to as “PAT”) apparatus is known as one of apparatuses for visualizing in-vivo information. In the measurement using a PAT apparatus, a tomographic image can be obtained by measuring the intensity and the time of generation of a photoacoustic signal emitted from a substance (optical absorber) that absorbs light in an object to be measured when the object is irradiated with light, and computing a distribution of the substance in the object.
A substance that absorbs light and emits an acoustic wave in a living body may be used as an optical absorber. For example, a blood vessel or a malignancy in the human body may be used as an optical absorber. In addition, for example, dyes that absorb light in the near-infrared wavelength region may be administered into the body and used as contrast agents. Light in the near-infrared wavelength region has little influence on the human body when the human body is irradiated with the light and has a high permeability to a living body. Thus, dyes that absorb the light may be suitably used as contrast agents for use in PAT apparatuses and fluorescence apparatuses.
To accumulate dyes that absorb light in the near-infrared wavelength region at measurement sites, such dyes are often entrapped in particles and used as contrast agents. Journal of Photochemistry and Photobiology B: Biology, 74 (2004), 29-38 (hereinafter, referred to as “Non-Patent Document 1”) discloses particles of a copolymer of lactic acid and glycolic acid (poly(lactide-co-glycolide), hereinafter, also referred to as “PLGA”), the particles containing indocyanine green (hereinafter, also referred to as “ICG”) and being prepared by an emulsion solvent diffusion method with polyvinyl alcohol (PVA) as a surfactant.
In the case of measurement with a PAT apparatus using a contrast agent entrapping a dye in particles, the dye in the contrast agent may be entrapped in high density. This is because a larger amount of the dye entrapped in the particles results in a higher molar extinction coefficient per unit particle and a higher photoacoustic signal.
For the ICG-containing particles disclosed in Non-Patent Document 1, the dye may leak from the particles in serum. This may be because ICG has a hydrophilic structure and a hydrophobic structure and is surface active. That is, in the ICG-containing particles disclosed in Non-Patent Document 1, ICG is more abundant on surfaces of the particles than in hydrophobic core portions of the particles. When ICG is mixed with serum, ICG present on the particle surfaces may interact with proteins in the serum to leak outside the particles.